Autothermal Fixed-bed Reactor Research Articles

Process Intensification through Directly Coupled Autothermal Operation of Chemical Reactors

Summary Autothermal operation of a chemical reactor involves coupling exothermic and endothermic chemical reactions for the purpose of thermal management without resorting to external energy sinks or sources. Often this is accomplished through regenerative or recuperative heat exchange between spatially or temporally separated exothermic and endothermic reactions. However, it is also possible to directly couple these reactions simultaneously within the same reactor volume, eliminating the heat-transfer bottleneck that characterizes much of chemical manufacture. It is not widely recognized that directly coupled autothermal operation allows dramatic process intensification. This perspective defines autothermal operation and contrasts it with conventional heat transfer for thermal management of chemical processes. The intensification and cost savings that can be achieved are quantified, and the implications to modular chemical process intensification are presented. Guidelines are proposed for designing directly coupled autothermal processes. Diverse examples are presented. Several challenges to expanding the field are critically discussed.

Oxidative Pyrolysis of Polystyrene into Styrene Monomers in an Autothermal Fixed‐Bed Catalytic Reactor

Styrene monomer recovery: A selective stream of styrene from fast pyrolysis of polystyrene is obtained with an autothermal fixed-bed reactor containing noble-metal (Rh, Pt) catalysts. Autothermal fast pyrolysis of polystyrene is an efficient means of recovering monomers from waste polystyrene in a continuous, high-throughput process without external heating.

Catalytic hydrolysis of sodium borohydride in an auto-thermal fixed-bed reactor

Catalytic hydrolysis of sodium borohydride is a promising technology for hydrogen storage. In this study, we report the performance of Millennium Cell Hydrogen on Demand TM ( HOD TM ) hydrogen storage system using a supported catalyst. Dynamic reactor startup and response characteristics were measured using a fixed-bed catalyst reactor at various hydrogen demand conditions. The catalyst reactor was operated without need of fuel preheating. Catalyst durability of more than 700 h was demonstrated with a sodium borohydride conversion over 95% at a liquid feed hourly space velocity (LHSV) of about 15 h - 1 and under a system pressure of 483 kPa.

State profile estimation of an autothermal periodic fixed-bed reactor

In this paper, the state profile estimation of a novel autothermal fixed-bed reactor (called circulation loop reactor, CLR), which can be operated as an autonomous periodic system, is studied. Since in autothermal operation the reactor shows sustained large oscillations, its dynamics have to be modelled as a strongly nonlinear distributed parameter system. The generally used lumping techniques (e.g. the orthogonal collocation method) for designing the state estimator of fixed-bed reactors are not applicable in this case. An approach for estimating the state profiles of the CLR based on a nonlinear distributed parameter observer is presented. To decrease the computational effort of the observer in real-time applications, the reduction of the detailed model of the CLR is investigated. In order to achieve asymptotic stability and fast convergence of the observer, the observability of the CLR is analysed, and use of features of the sustained oscillatory system and physical insight are made. A method based on exponential weighting functions is proposed to approximate the observation error profile. The temperature and concentration profiles are estimated from a small number of temperature measurements. The obtained observer was successfully tested with the dynamic simulator DIVA in the cases of errors in the initial condition and the model parameters, random noisy measurements, and unknown disturbances, also including the different choice of sensor locations, and different observer parameters. It shows excellent dynamic tracking of the reactor profiles, strong robustness, and easy tuning. It forms a basis for developing appropriate control strategies of the CLR.

State Estimation of a Distributed Parameter Fixed-Bed Reactor with Oscillations

Abstract In this paper, the state estimation of a novel autothermal fixed-bed reactor (called circulation loop reactor, CLR) is studied. Since in autothermal operation the reactor shows sustained large oscillations and has to be modelled as a strongly nonlinear distributed parameter system, the generally used linearization and lumping techniques (e.g. the orthogonal collocation method) for designing the state estimator of fixed-bed reactors are not applicable. An approach for estimating the state profiles of the CLR based on a nonlinear distributed parameter observer is presented. A simplified model for the CLR is derived, which reduces the computational effort of the observer in realtime applications. In order to achieve asymptotic stability and fast convergence of the observer, the observability of the CLR is analysed. Features of the sustained oscillatory system as well as physical insight are used for the observer design. A method based on exponential weighting functions is proposed for the approximation of the observation error profile. The temperature and concentration profiles are estimated from a limited number of temperature measurements. The obtained observer was successfully tested with the dynamic simulator DIVA in the cases of errors in the initial condition, random noisy measurement, and different sensor locations. It shows excellent dynamic tracking of the reactor profiles, strong robustness, and easy tuning. It forms a basis for the development of control strategies for the CLR.

On controlling an autothermal fixed-bed reactor at an unstable state-II: Discrimination among rival models to achieve suitable internal structure

Shortcomings of the traditional approach to modeling fixed-bed reactors are presented. These include a very oscillatory approximation to the initial dead time in the temperature response to boundary forcing, as well as the extreme resistivity of the large-scale lumped system to model reduction by modal techniques. These problems are shown to be directly related to an ill-conditioned internal structure of the approximate o.d.e. model, as a result of both an inadequate formulation of the original p.d.e. model and the discretization technique used in lumping the axial coordinate. A new technique for structural dominance analysis is described and shown to be well-suited for investigating the characteristics of a large-scale state-variable model. Five alternative lumped models of an autothermal tubular reactor are analyzed with the new technique; dispersive models are found to be well-behaved with respect to the shortcomings mentioned above. Finally, a set of guidelines is proposed for modeling fixed-bed reactors when an accurate low-order linear model of the system is desirable as, for example, in control applications.

On controlling an autothermal fixed-bed reactor at an unstable state—1: Steady-state and dynamic modeling

Models of distributed chemical reaction systems which are both accurate and wieldy are extremely difficult to obtain. This paper discusses traditional methods for deriving models of fixed-bed reactors, i.e. where the attempt is made to accurately account for steady-state and dynamic characteristics only. It illustrates the development of such models for the complex case of a tubular autothermal reactor with internal countercurrent heat exchange which exhibits multiple steady states for certain ranges of operating conditions. A two-dimensional dynamic model of the reactor is constructed, and then simplified to a linear state-variable form suitable for dynamic analysis by using successively: (i) a double collocation procedure to discretize the equations in both the radial and axial directions; (ii) a linearization technique around a specified steady-state profile; and (iii) the assumption of quasi-steady state for the coolant temperature and for the concentration of the reacting mixture. The dynamic analysis includes evaluation of the eigenproperties of the linearized model. The degree of instability of the reactor is correlated to some design variables and properties of the reaction. The sensitivity of the model to changes in parameters or operating conditions is also investigated. Finally, a shortcoming of the traditional modeling approach with its total emphasis on steady-state and dynamic properties is discussed. It is noted that the internal structure of a model also must be considered carefully if it is to be useful for controller design. A sequel paper discusses this virtually unrecognized point, again using the autothermal reactor system for example purposes.

On controlling an autothermal fixed-bed reactor at an unstable state—III: Model reduction and control strategies which avoid sstate estimation

A simplified approach for the control of fixed-bed reactors is discussed. Key elements include: (i) careful modeling of the reactor to obtain a lumped approximation with good internal structure (i.e. appropriate coupling between inputs and state variables through the different response modes); (ii) a major reduction in model size to an accurate, low-order form; (iii) derivation of a control algorithm for the low-order model; and (iv) direct application to the full-order system without the use of state estimation. The success of this approach is rooted in the choice of modeling techniques and of a model reduction procedure. The powerful new reduction method of Litz is evaluated here. Applications of the technique to a simulated unstable autothermal reactor are demonstrated for the case of modal control with incomplete state feedback. Results point to a high probability of success in subsequent experimental tests.

On controlling an autothermal fixed-bed reactor at an unstanble state—IV: Model fitting and control of the laboratory reactor

This study describes in detail model fitting and experimental control implementation for a tubular autothermal reactor running the water-gas shift reaction. The reactor is interfaced to a real-time computer for data logging and control purposes. Certain unknown reaction and heat transport parameters are either evaluated through indirect measurements or estimated from steady-state temperature and concentration data. The heat capacity of the catalyst bed is then adjusted so as to improve the agreement between simulated and measured transient responses. The control approach is purposely kept very simple: measurement of only a few bed temperatures, no state estimation scheme, and use of a single manipulated input. Operation of the reactor at an unstable steady state is clearly demonstrated by several independent tests. Finally, excellent agreement between experimental and simulation results are observed for various cases of pulse perturbation of the heat input at the catalyst bed entrance.